Testing & Code Quality

Mocking & Test Doubles

Mocks, stubs, fakes, spies, and fixtures replace real collaborators so tests can control inputs, isolate behavior, avoid slow dependencies, and observe important interactions.

A stub returns controlled data
A fake has working simplified behavior
A spy records interactions
A mock encodes expectations
Mock at system boundaries, not every function call

Test doubles
Double	Purpose	Example
Stub	Return predefined value	fake API response
Fake	Simplified working implementation	in-memory repo
Spy	Record calls	was callback called?
Mock	Pre-programmed expectation	must call save once
Fixture	Reusable test data	sample user object

Fake repository

function createFakeUserRepo() {
  const users = new Map()
  return {
    async save(user) {
      users.set(user.id, user)
    },
    async findById(id) {
      return users.get(id) ?? null
    },
  }
}

A fake can support several tests without making every interaction brittle.